Armour, in most cases, wasn't really intended to prevent injury. Although, armour could stop a glancing blow or seriously negate some weapon strikes it was mostly to give the wearer a better chance of preventing death. Shields and foot work were used to prevent injury more than anything as I understand it.
Your understanding is completely and totally wrong. (no insult intended), armor worked to both lessen the impact of injuries (turning a potentially crippling injury into a minor inconvenience) and to completely negate strikes entirely. A good suit of platemail renders one almost impervious to sword strikes and highly resistant to arrows and even early firearms. It required either the heft of a polearm or heavy mace\axe to cause serious injury to somone in full plate, or the precision of a dagger to strike at the weak points.
Right up at the end of the medieval period just before muskets were becoming common, heavy armor was nigh impregnable against most forms of assault, armor ruled the battlefield. The introduction of the musket (the first firearm that could reliably penetrate hardened steel armor at anything other than close range) started the end of the plate armor era.
The gun, however, changed things substantially, ever since the gun was invented our offensive abilities have outpaced our defensive ones. Modern armor serves an entirely different function to ancient armor (that of reducing the impact of glancing blows and shrapnel, rather than protecting the wearer in straight-up combat.)
I feel i should also add that modern Interceptor Body Armor is serious stuff and the vests the US armoy use are rated to stop 3 consecutive hits from a 7.62×51 NATO AP round at a range of 10 meters. Reports from Iraq and Afghanistan suggest that the plates can take upwards of 7 consecutive hits without being penetrated. Against somone in modern ballistic armor, you're better off stabbing him or aiming for an area not protected; unless you're packing some serious firepower you probably won't get through the armor before he fills you full of holes.
Interestingly, we're seeing Chainmail make a return in modern armor in anti-stab vests, as it's far more effective than standard ballistic weaves against ice pick and knife attacks, while retaining the flexibility that ceramic plates lack.
Here's a link to the Wikipediar article on ballistic vest performance standards; it's an interesting read. Note that the US army issues Type III vests.